Various forms of cruise control systems are known. According to a typical cruise control system for an automobile, after a desired vehicle speed has been reached by manual operation of the throttle pedal, the set switch is activated and the actuator takes over the control of the throttle pedal and automatically maintains the vehicle speed at a constant level without requiring any efforts on the part of the driver.
The actuator for such a cruise control system may consist of a vacuum actuator using engine vacuum or a motor driven actuator. A motor driven actuator is highly compact and is capable of performing an accurate control action. Typically, a transistor bridge circuit is used to selectively drive the actuator in either direction according to the need to accelerate or decelerate the vehicle.
Such a drive circuit is required to be highly reliable and is desired to be equipped with various protective and fail-safe features. For instance, a CPU consisting of a micro processor used in the control unit for a cruise control system is normally provided with a watch dog timer circuit which keeps producing a certain steady pulse signal as long as the CPU is functioning normally, but produces a certain abnormal DC signal or a high frequency signal when the CPU has ceased to function normally. The abnormal output of this watch dog timer circuit is typically used to disconnect the electromagnetic clutch provided in the output shaft of the actuator to terminate the action of the cruise control.
However, when the CPU stops functioning normally, it could produce confused control signals to the drive transistors and, depending on the combination of the confused control signals, some of the drive transistors may short-circuit and may be destroyed in a very short time. Therefore, it is desirable to provide a protective circuit to such a transistor bridge circuit and an example of such a protective circuit is disclosed in Japanese patent laid-open publication No. 57-119682. According to this disclosed protective circuit, a certain interlock circuit is provided to each pair of transistors which are not desired to be in conductive states at the same time so as to prevent them from being brought into conductive states at the same time. However, this solution involves the use of power transistors for conducting large electric current and this prevents compact design of the system and causes an increase in the manufacturing cost.
Furthermore, even when no short-circuiting takes place, it is desirable whenever the CPU produces a confused output to disconnect the actuator or to drive the actuator in the direction to decelerate the vehicle according to the principle of fail-safe. However, the above mentioned solution does not offer this advantage.
Typically, a cruise control is terminated when a brake pedal is pressed, when a clutch pedal is activated, and when a transmission gear is shifted. Additionally, as safety features, it is desirable to terminate the cruise control and/or to drive the actuator in the direction to decelerate the vehicle when the rotational speed of the engine has exceeded a certain limit, and when the vehicle speed has fallen below the target vehicle speed beyond a certain limit, and when any abnormal condition in the control circuit is detected. However, depending on the speed of the activation of such protective circuits, electric current may continue to be supplied to the drive circuit for a time period between the time point when the cruise control is activated the time point when the cruise control is terminated upon detection of any abnormal condition and this could cause destruction of a part of the circuitry or make the existing abnormal condition even worse.
Therefore, it is highly desirable to run a self diagnostic routine before the cruise control is activated in view of improving the fail safe features of the cruise control system.
In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle cruise control system which fails in a safe manner whenever the CPU of the cruise control system produces a confused output.
Another object of the present invention is to provide a vehicle cruise control system which is reliable and permits a compact design.
According to the present invention, these and other objects of the present invention can be accomplished by providing a vehicle cruise control system, comprising a control circuit for producing a control signal for maintaining a speed of a vehicle at a fixed level according to a difference between an actual vehicle speed and a target vehicle speed, and a drive circuit for producing a drive signal for selectively driving an actuator in a direction either to accelerate the vehicle or to decelerate the vehicle according to the control signal from the control circuit, wherein: the drive circuit is provided with a transistor bridge circuit comprising four drive transistors for producing the drive signal while the control circuit is provided with a detection means for detecting an abnormal state of the system and an inhibiting means which brings at least one of the drive transistors into a non-conductive state when any abnormal state is detected by the detecting means.
Thus, even when the control circuit produces a confused control signal, the detection means forces one of the drive transistors, typically one of the acceleration drive transistors, into a non-conductive state and not only the short-circuiting of the drive transistors is avoided but also the system is allowed to fail in a safe manner. One of the deceleration drive transistors may be brought into either a conductive state or non-conductive state.
The detection means may comprise a circuit for monitoring the state of one of the drive transistors or a watch dog timer circuit for monitoring the action of a micro processor in the control circuit.
According to a certain aspect of the present invention, the drive circuit comprises an acceleration drive circuit and a deceleration drive circuit, and the acceleration drive circuit is connected to a power line by way of a brake switch which opens upon pressing of a brake pedal while the deceleration drive circuit is directly connected to a power line as a favorable feature for permitting the system to be shut off and/or to decelerate the vehicle in a reliable manner.
According to another aspect of the present invention, by sending a signal to one of the drive transistors to close it after a power switch of the vehicle cruise control system is turned on and before the set switch is activated, the opening capabilities of the transistors which are connected in series with the said closed drive transistor can be tested.
In a vehicle cruise control system having a lower limit switch for disconnecting the motor when a limit of a stroke of the motor to decelerate the vehicle has been reached, by sending a deceleration signal to the transistor bridge circuit after a set switch for initiating a cruise control is activated and before the electromagnetic clutch is connected, the opening capabilities of the drive transistors located in a path for accelerating the vehicle can be tested. If an acceleration signal is sent to the transistor bridge circuit, the opening capabilities of the drive transistors in the path for decelerating the vehicle can be tested. Further, by sending a boosted acceleration signal for taking up slack in the linkage existing between the working end of the actuator and a speed control means of a vehicle engine immediately after the electromagnetic clutch is connected, the closing capabilities of the drive transistors located in a path for accelerating the vehicle can be tested.
The opening capability of the transistor for driving the electromagnetic clutch can be tested by detecting electric current conducted by the transistor after the set switch for initiating the cruise control is activated and before the electromagnetic clutch is connected, and the closing capability of the transistor for driving the electromagnetic clutch can be tested by detecting electric current conducted by the transistors after the electromagnetic clutch is connected.